Two girls on a train try to count how often trains run — and disagree
Two schoolgirls ride a suburban train to their dacha. Every 5 minutes, a train passes going the opposite way. One girl says that means 12 trains leave the city every hour. The other disagrees. Trains run equally often and evenly spaced in both directions — so how many actually leave each hour?
Reveal the answer
Six. Because the girls' own train is moving toward the oncoming ones, it meets a train every 5 minutes — but that's the combined closing speed of both trains, not one train's real spacing. Each direction's trains are really 10 minutes apart, which works out to six departures an hour, not twelve.
— Boris Kordemsky, The Moscow Puzzles: 359 Mathematical Recreations — Problem 63
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